This invention relates to thermosettable resin compositions. In one embodiment, the invention relates to copolymers of a 1,2-dialkylidenecyclobutane and a bisimide. In a preferred embodiment, the invention relates to toughened bismaleimide resins.
Advanced composites are high-performance materials made up of a fiber-reinforced thermoplastic or thermosettable material. Thermosettable materials useful in advanced composites applications must meet a set of demanding property requirements. For example, such materials optimally have good high-temperature properties such as high (above 200.degree. C.) cured glass transition temperature and low (less than 3%) water absorbance at elevated temperature. Such materials also exhibit high toughness, as reflected in a Mode I fracture toughness above 2 MPa.m.sup.1/2. For ease of processing in preparing adhesives and prepregs for composite parts, the uncured material will ideally have a low (below 120.degree. C.) melting temperature.
Examples of thermosettable materials useful in advanced composites include epoxy resins and bismaleimide resins. Epoxy resins have good processing properties, but generally have relatively low glass transition temperatures and unacceptable high-temperature water absorbance.
Bismaleimide resins have superior high-temperature properties but are very brittle and further tend, because of their high softening points, to require solvents in order to be readily processable. In addition, standard cured bismaleimide resins tend to have high (in the 5-7% range) 93.degree. C. water absorption. Standard olefinic chain extenders for bismaleimides (such as bis-allylphenols) give rates of chain extension which are not significantly higher than the rates of the subsequent crosslinking reactions, making it difficult to use such materials to obtain the high molecular weight between crosslinks needed for very high toughness. Certain bisbenzocyclobutene crosslinkers for bismaleimides give high toughness, but these monomers are quite expensive to prepare, involving multistep syntheses from the nearest commercially available material.
It is thus an object of the invention to provide new thermoset resin materials. In one aspect, it is an object of the invention to provide comomomers which provide low-melting bismaleimides which cure to high-Tg, tough resins having low water absorbance.